There are some obvious answers: Herbert & Son's Dune, Hamilton's _Anita Blake_ books. Some arguable cases: Butcher's _Dresden Files,_ Weber's Honor Harrington books, or perhaps Brust's Jhereg novels.
The one that came to my mind that nobody else suggested here was Spider Robinson's Callahan series, which went from joyfully interesting, human, creative, and fun, to a simply self-indulgent and paycheck-cashing travesty of its original self.
Which book series went on WAY past its expiration date?
The best science fiction and fantasy books are so addictive, you finish one book and immediately want to reach for the next. Until… they start to go downhill. Because the Law of Diminishing Returns applies to every universe. What’s the book series that most outstayed its welcome?
Oh, gosh. Dune, Pern, Xanth, Gor… I'd have to actually think for a minute to come up with others.
As bad as this trend is in SF books, it's even more acutely present in fantasy reading, TV series, and, perhaps most egregious of all, pop-Anime.
Ah, yes. I've seen brilliant anime short series get rebooted a dozen times, losing something interesting each time.
Hmmmm. Which now makes me think of Scooby-Doo — though, honestly, some of the more recent reboots have been pretty good.
+Scott Randel Dune sounds like a classic example — I've never read any Junior's titles, but have never heard anything good about them. I got tired of Xanth after the fourth or fifth — and I understand he's still cranking them out. I barely made it through a single Gor novel.
Hmmm. Outside the SF/Fantasy realm, I'd throw in Robert Parker's Spenser series. Each new installment was a shorter read, and pleasantly comfortable, but with all the emotional churn and suspense of a carton of yogurt, quite different from the early-days novels.